A simple photon mapping based ray tracer developed to support experiments with
nearest-neighbour algorithms.

Features:
* Lua scene and view definitions.
* Sphere, plane and triangle mesh primitives.
* Point and rectangular light sources.
* Soft shadows.
* Photon mapping.

To do:
* Proper sampling for initial rays.
* Mode to only use photon mapping for indirect lighting.
* Look at ray propagation for Lambertian materials.
* Look at separate caustic photon map.
* Spherical light sources.
* Textures.
* Take another look at dielectric implementation.

When I first started working on this in 2010/2011, the design and
implementation were influenced by the following books:

Ericson, C. (2005) Real-Time Collision Detection, Morgan Kauffman,
San Francisco, CA

Jansen, H. W. (2001) Realistic Image Synthesis Using Photon Mapping,
2nd Edition, A K Peters, Natick, MA

Shirley, P. and Morley, R. K. (2003) Realistic Ray Tracing, 2nd Edition,
A K Peters, Natick, MA

When I started working on this again in 2018, the following books were quite
helpful:

Shirley, P. (2016) Ray Tracing in One Weekend
Shirley, P. (2016) Ray Tracing: The Next Week
Shirley, P. (2016) Ray Tracing: The Rest of Your Life